Page 50 - Adams and Stashak's Lameness in Horses, 7th Edition

16 Chapter 1

A fine terminal branch of each palmar digital nerve
and an accompanying small artery constitute a neuro­
VetBooks.ir venous plexus Digital vein membrane of the distal interphalangeal joint to enter the
Coronary
vascular bundle that descends adjacent to the synovial
distal phalanx.
5
Bulbar vein
Additional cutaneous innervation of the fetlock is
supplied by terminal branches of the medial cutaneous
Parietal antebrachial nerve dorsomedially and the dorsal branch
venous plexus of the ulnar nerve dorsolaterally. Medial and lateral pal­
mar metacarpal nerves emerge immediately distal to the
Solar distal end of the respective small metacarpal (splint)
venous plexus
bone, supplying branches to the fetlock joint capsule
and ramifying in the superficial fascia of the pastern. It
Digital vein has been reported that in some instances, a terminal
branch from the medial palmar metacarpal nerve
descends to the coronary band (Figure 1.14). 19,31 An
occasional variant, a palmarly directed branch from the
Bulbar vein
medial palmar nerve in the distal metacarpus, courses
palmar to the medial palmar digital nerve, reaching the
digital cushion (Figure 1.14). Another variant branch
may arise from the lateral palmar nerve in the proximal
metacarpus, cross over the fetlock, and extend obliquely
to the coronary band (Figure 1.15).
Electrophysiologic studies confirm that stimuli on the
Figure 1.16. Venogram of equine foot. Source: Photo courtesy medial half of the digit and fetlock of the forelimb are
of Dr. Andrew Lewis. mediated by the median nerve and stimuli on the lateral
half are mediated by the median and ulnar nerves. 5
veins and in the bulbar veins and their branches. Thus, Although direct communication between the distal
the flow of blood may take different routes with the interphalangeal joint and the navicular bursa is very
weight‐bearing force essential to its proximal flow. 26 rare, indirect communication via diffusion of molecules
has been demonstrated. Dye injected experimentally
8
into the distal interphalangeal joint diffused into the
Nerves of the Digit and Fetlock
navicular bursa and also stained the synovial coverings
As they descend to the fetlock, the medial and lateral of the CSLs and the distal sesamoidean impar ligament
palmar nerves supply small branches to the fetlock and and the medullary cavity of the navicular bone.
flexor tendons. Each then gives off a dorsal branch at
this level, continuing over the widest part of the joint as
the medial and lateral palmar digital nerves (Figures 1.14 Basic Functions of the Digit and Fetlock
and 1.15). The corresponding digital artery lies between In the standing position, the fetlock and digit are pre­
this dorsal branch and the palmar digital nerve. The dor­ vented from nonphysiologic hyperextension by the sus­
sal branch courses distad between the digital vein and pensory apparatus of the fetlock (interosseus muscle
artery, branching midway down the pastern. In approxi­ [suspensory ligament], palmar ligament and proximal
mately one‐third of the cases, an intermediate branch sesamoids, and distal sesamoidean ligaments), digital
also arises from the dorsal aspect of the palmar digital flexor tendons, and collateral ligaments of the joints.
nerve. The dorsal (and intermediate, when present) During flexion and extension of the digit, most of the
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branches supply sensory and vasomotor innervation to movement comes from the fetlock joint. The least
the dorsal part of the fetlock joint, dorsal parts of the amount of movement is in the pastern joint, and move­
interphalangeal joints, coronary corium and dorsal parts ment in the coffin joint is intermediate. Although the
of the laminar and solar coria, and dorsal part of the pastern joint is a hinge joint, normally providing only
cartilage of the distal phalanx. 5,12 limited flexion and extension, manipulation can pro­
The main continuation of the palmar digital nerve duce transverse movement and some rotation when the
descends palmar and parallel to the digital artery. The joint is flexed.
nerve and artery are deep to the ligament of the ergot as When the unshod hoof contacts the ground, the heels
the latter descends obliquely across the lateral aspect of usually strike first, followed in sequence by the ground
the pastern. A branch may arise from the lateral palmar surfaces of the quarters and toe. Expansion of the heels
digital nerve and perforate the lateral ligament of the is facilitated by the elasticity of the hoof wall. Most of
ergot (Figure 1.15). the impact is sustained by the hoof wall, and compres­
The palmar digital nerves supply the palmar parts of sion of the wall creates tension on the interlocking epi­
the fetlock joint capsule and of the digit: skin, pastern dermal and dermal laminae and, hence, to the periosteum
joint capsule, digital synovial sheath and flexor tendons, of the distal phalanx. The concave sole does not absorb
distal sesamoidean ligaments, coffin joint capsule, navic­ much force although it is depressed somewhat by the
ular bone and its ligaments, navicular bursa, palmar part downward force of the distal phalanx, causing expan­
of the cartilage of the distal phalanx, part of the laminar sion of the quarters. Descent of the coffin joint occurs as
corium, coria of the sole and frog, and digital cushion. the navicular bone moves in a distopalmar direction,